1. Field of the Invention
The present invention relates to a temperature compensated crystal oscillator circuit. More particularly the present invention relates to a temperature compensated crystal oscillator circuit that utilizes switched capacitor technology wherein all circuitry is contained in a single electronic package, does not need to be calibrated after it leaves manufacturing, and that provides an oscillation frequency with little variation over a wide temperature range.
2. Description of the Related Art
Keeping track of time electronically is an important function. Real time clock circuits have become more and more important in the electronic design industry. Portable computers, portable phones, portable global positioning devices, and many other electronic devices which require accurate real time clock circuits are being used in a wide variety of environments. The electronic devices are required to operate in a cold environment one day and then the next day the same electronic device will be used in a warm environment.
It is well known that temperature has an effect on real time clock circuitry. More particularly as temperature changes the frequency of an oscillation circuit also changes. A crystal, for example a tuning fork style crystal, oscillates at a higher frequency in a midrange temperature and at a lower frequency in cold and a hot temperatures. An AT-cut style crystal will oscillate at a lower frequency in a warm temperature than in a cold temperature. Designs for a real time clock or any oscillation circuits have allowed for speeding up and slowing down of an oscillator in high and low temperatures with a belief that the circuit will spend most of its time in moderate temperatures where the variance in oscillation frequency rate is small. It was also thought that over the course of a year the circuit may spend about the same amount of time in a cold environment as a hot environment thereby nearly balancing the temperature related increase and decrease in oscillation speed.
Compensation circuits used in conjunction with oscillation circuits speed up or slow down the oscillation circuit so that oscillation output is at more of a steady frequency over a wider temperature range. In the past, thermistor and analog temperature sensing devices were used to sense temperature changes.
There are many drawbacks and pitfalls in the electronic designs of former temperature compensated oscillators. First since an analog temperature sensing device is a non-linear device, it was difficult to calibrate the compensation circuitry, especially in mass produced devices. Second, the temperature sensing was not done in the same location as the crystal oscillator was located on the circuit board. There are many temperature gradients around circuitry operating on a printed circuit board. Components on the printed circuit board operate at different temperatures thereby creating temperature gradients. When a temperature sensor is not physically and substantially a part of the oscillation circuitry, the sensor may be sensing a temperature gradient from a neighboring part and not the temperature of the crystal oscillator. Thus, the result of the former temperature compensation circuits is an inaccurate oscillation frequency output. The result of products incorporating the prior temperature compensating circuitry wherein the temperature sensor was not physically part of the compensation and oscillation circuits required the finished products to have the oscillation circuits calibrated after the product was finished being manufactured.
Thirdly, previous crystal oscillators that have been temperature compensated still have a substantial output frequency deviation over the specified temperature range. Previous compensated devices have had frequency deviations over a temperature range of 0.degree. to 40.degree. C. of more than 20 ppm or more than 10 minutes per year. These deviations have become unacceptable in many areas of electronic design such as portable computers, cell phones and medical equipment.
There is a need for a temperature compensated oscillation circuit that maintains better accuracy than prior compensated circuits over a defined temperature range; that incorporates accurate temperature sensing of the area about the crystal oscillator; that is provided in a single electronic package; that is less expensive to manufacture and sell than prior circuitry for compensating a crystal oscillator over temperature; and that is pre-calibrated so that it can be installed in an electronic device without being re-calibrated to the environment within the device.